Match List-I (Test / Process) with List-II (Main outcome measured) and select the correct code:

Question

Match List-I (Test / Process) with List-II (Main outcome measured) and select the correct code:

List-I

i. BOD5

ii. COD

iii. DO measurement

iv. SVI

List-II

a. Available Oxygen

b. Aerobic biodegradable load

c. Total COD

d. Settling ability

Choose the correct match:

1. A. i-b, ii-c, iii-a, iv-d (Correct answer)
2. B. i-c, ii-b, iii-d, iv-a
3. C. i-b, ii-a, iii-c, iv-d
4. D. i-d, ii-c, iii-b, iv-a

Correct Answer

Option A — i-b, ii-c, iii-a, iv-d

Detailed Solution & Explanation

The correct answer is i-b, ii-c, iii-a, iv-d.

Key Points

• Four routine determinations, each with a distinct outcome:
  • i. $\text{BOD}_5$ - b. Aerobic biodegradable load. Oxygen consumed by micro-organisms over five days at 20°C.
  • ii. COD - c. Total COD. Oxygen demand of essentially all organic matter, measured by chemical oxidation.
  • iii. DO measurement - a. Available oxygen. The oxygen actually dissolved and available to aquatic life at that moment.
  • iv. SVI - d. Settling ability. How well activated sludge compacts after 30 minutes.

Additional Information

• The distinction between BOD/COD and DO is the conceptual heart of this question. BOD and COD are measures of demand — how much oxygen the water *will* consume. DO is a measure of supply — how much is present right now. Confusing demand with supply is the standard error.
• DO is measured either by the Winkler (iodometric) titration or, in practice today, by a membrane electrode or optical probe.
• A healthy river typically holds above 5-6 mg/L; below about 4 mg/L most fish are stressed, and at zero the water is anaerobic.
• SVI is the only one of the four that describes a physical property of sludge rather than a chemical or biochemical demand, which makes it the easiest pairing to secure first.